Power amplifier



April 26, 1960 E. s. MILLER :T AL 2,934,714

POWER AMPLIFIER Filed Aug'. 2s, 1957 4 Ir I mx. i' 6 AIO Pc, Iv' l 1 A 4 n a' Abms- 15mn.

r.-e-o f- L /eo' E @Y i i fr fm@- PAf-AM F .5r/165iL ,WIJZE "NDL Y ANLR auf amro H12/1X7) TUBES HlzAv?) 7565 ze" Z .AE :11BZ 40 j United States Patent() i 2,934,714 POWER AMPLrarER Edward S. Miller, Highland Park, and Robert M. Grodinslky, Deerfield, Ill., assignors to Sherwood Electronic Laboratories, Inc, Chicago,'lll., va corporation of Illinois This invention relates to power amplifiers, particularly, but not necessarily, to electron tube power amplifiers used in high fidelity sound amplifierequipment, lwherein one of the prime considerations is the provision of power output with'minimum distortion.

In recent years, the demand for high fidelity sound equipment has greatly increased, particularly so inmiddle income families. The cost of such equipment is, therefore, avery important consideration, and sound'equipment manufacturers are continuously seeking Vways and means of reducing costs without, however, sacrificing Vthe high fidelity characteristics of the equipment.

In addition to the obviously desirable characteristic of wide. frequency response, true high fidelity requires low distortion at high power output levels and low hum or noise levels. The output capabilities of high fidelity Ysound equipment are generally specified in terms of maximum watts outputV where the distortion level is ator below a specified limiting distortion level. To increase the wattage output of the amplifier without increasing the distortion level has heretofore required a significant increase in cost of the equipment brought about by either the use of more expensive electron tubes with higher power output capabilities and/or the addition of various other components which reduce distortion.

Before discussing the objects and advantages of the present invention, a brief discussion of the factors affecting distortion will be given. Where a single tube power amplifier circuit is utilized, it is apparent that the driving of the power amplifier tube far into its lower non-linear regions of operation will result in substantial distortion in the output signal waveform. The degree to which such a tube can vbe driven without exceeding a limiting distortion level is obviously tied in very directly with the length of the linear and near-linear portions of its voutput current vs. signal input curve. When a single tube is being operated in its linear region of operation, it is operating class A, and, when it is driven to a point where the maximum negative swing of the signal drives it through its low non-linear region into a plate current cutoff condition, it is being operated class AB or class B.

Distortion increases as the operation of the tube is varied from a given class A condition toward a class AB or class B condition. An increase in the grid to cathode bias of a tube originallyoperating in a given linear or nearlinear class of operation will thereby shift the class of operation toward a class AB and more non-linear condition of operation.

The use of two power amplifier tubes connected in push-pull enables the tubes to be operated well into their non-linear regions, considering the tubes individually, while providing an over-all linear or low distortion youtput to input signal characteristic, considering the output across the secondary winding of the output transformer commonly used in such circuits.'A When the grid-toy `cathode bias conditions permit both tubes todraw platev current at all times and the input signal does not drive 2,934,714 Patented Apr. 2 6, 1960 the grids positive', the amplifier is said to ybe Voperating class A and with correspondingly relatively small distortion. When the bias level is such that the 'input signal drives the tubes to cut-off, theyare operating class AB or B. V As in the case'of single tube operation, it is a valid generalization to state with a push-pullamplifier. that distortion increases as the condition of the tubes is varied from a given class A operating condition toward afclass AB orBoperating condition,'obtained, for example, by` an increase Vin the grid-to-cathode bias ofthe tubes.

In both Vsingle tube and push-pull p'owerramplifier circuits, the average'value of plate current increases-from a no-signal to a Asignal condition if the tubes, considered singly, are driven into their lower non-linear regions of operation. If ordinary cathode bias is utilized, wherein they bias voltage is developed solely'by the flow of plate current through a cathodecircuit impedance, then-the resulting shift of cathode bias under a signal condition resultswin a shift from a theoretical classl of operation, based on the no-signal 4cathode bias, toward or farther into a class AB or class Bv state of operation with a consequent increased distortion r for a given input signal. 'Ihis `fact has been well known, and variousvmeans, therefore, yhave been heretofore developed for stabilizing the bias of the power output tubes of high-fidelity amplifiers. The present invention yoffers a new and improved circuit for stabilizing the cathode bias.

' As indicated above, in addition to the problemrof output distortion, the problem of hum or noise, level-is also an important consideration in high fidelity sound equipnient.' Most of the hum or noise originates in the preamplifier stages of athigh fidelity amplifier, since the noise Aoriginating therein is amplified to the greatest extent. One source of noise in an amplifier is indirectly'due to the fiuctuating component of the current used to heat the filaments of electron tubes. Accordingly, the filaments, particularly of the preamplifier tubes. are sometimes heated by well-filtered direct current. In the interest of economy, it has heretofore been proposed to utilize the plate current of the power amplifier tube or tubes to heat `thefilaments of the pre-amplifier tubes. For best operation and long life, manufacturers of electron tubes usually recommend that the filaments be operated at certain current levels. If the current is increased significantly above the rated value, then the life of the tube is greatly affected, and, if the current is significantly below the rated amount, insuicientelectron emission will result with reduced amplification and increased distortion and unduly lengthened warm-up time. Usually, the plate current levels found in power amplifiers, particularly in the 20 watt and under amplifiers, were' significantly under the recommended current ratings of the tube filaments even when two push-pull connected tubes are used in the power amplifier stage, so that heating the tube filaments with such plate current resulted in a substantially less-than-ideal operation of the pre-amplifier tubes. Sometimes the filaments were damaged by excessivel plate current flow brought about by control grid emission.

An additional factor increasing warm-up time, is that the power amplifier tubefilaments must heat up before the plate current heating the pre-amplifier tube filaments reaches its normal value. It was not uncommon to have warm-up times of 20 seconds or more which was disturbing to many users of this equipment.

' It is one of the primary objects of the present invention to provide a power amplifiercircuit having a stabilized bias voltage in a novel and unique way and which, at the same time, enables the proper heating of 'the pre-amplifier tube filaments by a plate current of the power amplifier, even thoughthe plate current is not at or near the rated value of the filaments. A' further vof direct current voltage.

object of the invention is to heat the pre-amplifier filaments with plate current in a novel power amplifier cathode circuit designed to stabilize current ow through the vfilaments in spite' of wide fluctuations ina plate current. Another object of the present invention is to provide a high-delity sound amplifier which enables a substantial increase in wattage output thereof for a given limiting distortion level and with a low noise level, at a significantly lower cost than was heretofore possible using prior bias voltage stabilization and pre-amplifier filament heating techniques.

In its most preferredform, the present invention uti lizes-a push-pull arrangement of power amplifier tubes having a novel and improved cathode circuit design including the filaments of the pre-amplifier tubes and, in most cases, additional resistance means. VThe effective resistance of the cathode -circuit including said preamplifier filaments and resistance means is such that the current flowing through the filaments is less than the rated amounts therefor. Any deficiency in cathode bias and filament current is supplied by an external source This source of direct current voltage may include the output winding of a full Wave rectifier circuit used for providing plate or bias voltage for the tubes of the amplifier circuit. Usually, such a circuit uses a transformer where the center point of the secondary winding thereof is grounded. According to a specific aspect of the present invention, connections are made from the cathode circuit of the power amplifier tubes through respective rectifiers (preferably of the semi-conductor type, such as selenium rectifiers, because of their inexpensiveness and high current capacity for short time intervals) to points on the secondary winding of the rectifier circuit transformer spaced equidistant from and on opposite sides of the grounded center point thereof. The rectiers are similarly polarized to pass current in a direction which will send current through the cathode circuit in the same direction as the flow of plate current therein. Suitable filtering means are provided to lter out any significant ripple from the rectifier outputs. rThe filtering means preferably is a filter capacitor connected to the output of the rectifier, and a capacitor connected across the cathode circuit filaments. Most preferably, the tap-off points on the power transformer are made at points which will provide, after suitable filtering, a direct current voltage equal to the desired bias voltage for the power amplifier tubes. In such case, the above-mentioned rectifiers are con nected to the cathodes of the power amplifier tubes or to points near cathode potential so that voltage of the entire cathode circuit is stabilized at the desired level.

The resistance means in the cathode circuit is of such value and is so arranged that, at the voltage* applied to the cathode circuit by said external source of direct current voltage, the current fiowing through the filaments is near their rated value. Thus, with this arrangement, not only is stabilization of the bias of the power amplifier tube obtained, but the stabilization of the current fiow through the preamplifier filaments is obtained at the desired current rating thereof. When the amplifier is initially turned on, very little current is supplied to these filaments by the plate current of the power amplifier, and the required current will be provided from the external source of voltage. Since cold electron tube fila- -ments have a very low resistance relative to their normal hot resistance, relatively large currents are initially supplied through the semi-conductor rectifiers of the external voltage source. Such rectifiers have the advantage of high current capacities for short periods. The heating up of the preamplifier filaments need not wait, therefore, for the heating up of the power amplifier tubes, thereby substantially reducing the over-all warm-up time of the amplifier. Furthermore, the filaments are being operated at their rated values so that maximum life and optimum operation thereofY is ensured. Since, under normal op erating conditions, the selenium rectifiers supply only part of the current needed for the filaments, the semiconductor rectifiers may have a low current rating relative to the current required by the filaments.

In its broader aspect, the present invention contemplates the use of only a single power amplier tube rather than a push-pull arrangement of two tubes and also the use of only one rather than two filter capacitors. Also, theoretically at least, adirect current battery source for the external current-producing means may be used as the external voltage source. I n the latter case, a capacitor for filtering out audible frequencies from the pre-amplifier filaments would be unnecessary. 1t may, however, be desirable to include a capacitor in the cathode circuit for other reasons. Also, although the present invention has its most practical application where the plate current of the power amplifier tubes is substantially less than the rated current for the pre-amplifier laments to be heated thereby, the invention also has utility in reducing warm-up time and stabilizing cathode bias and filament current where the plate current is greater than the current required by these filaments.

For a fuller explanation of the present invention, reference should now be made to the specification and the drawings wherein:

' Fig. 1 shows a single tube power amplifier circuit and the output current vs. input current characteristic thereof;

Fig. 2 is a box diagram Vshowing stages commonly found in high fidelity and sound amplifiers;

Fig.'3 shows a single tube power amplifier'circuit which incorporates the novel and improved cathode circuit of the'present invention; andv Fig. 4 shows'a push-pull power amplifier which incorporates the presentinvention in its most Vpreferred form.

Refer now to Fig. l which shows a conventional type single tube power amplifier circuit and the tube characteristic thereof. As there shown, the power amplifier circuit includes an electron vacuum tube 2, which will be sometimes referred to as an electron tube device or means. The electron tube may be any one of a number of well known power amplier tubes, such as the type 6L6 beam power tube. Such tube has a plate electrode 2a, a control grid electrode 2b, a cathode electrode 2c, a screen electrode 2d, beam forming plates 2e, and filament 2f. The plate electrode 2a is connected through a.

step-down output transformer 4 whose secondary winding 4a is connected to a suitable speaker system 6. The primary winding 4b thereof is connected to a suitable source of positive direct current voltage 8 whose negative terminal isV usually connected to ground or some other reference or common point 10. The cathode electrode 2c is connected to the common point 10 through a cathode'resistor 12 shunted by a suitable filtering capacitor 14.l The input circuit of the tube extends from the control grid electrode 2b through a suitable resistor 16 to ground 10. A suitable source of driving voltage 18 is connected across the resistor 16, and is usually the output circuit of a control amplifier tube or a pre-amplifier tube. A voltage dropping resistor 20 is connected between the screen electrode 2d and the positive terminal of the source of direct current voltage 3, and an isolating or filter capacitor Z2 is connected between the screen electrode and ground. The filament 2f maybe connected to a conventional filament transformer.

A typical tube characteristic curve C for the power amplifier tube 2 is shown in Fig. 1. The line L1 therein represents vunits of the net voltage applied between the grid and cathode electrodes and the line LZ represents the output plate current for such voltage. When the output of the grid driving voltage source 18 is zero, the plate current will stabilize at a value I0 represented by the ordinate of point P-l on the characteristic curve C.

At that point, the grid to cathode voltage has a value e-o which is equal to the plate current times the resistance of cathode resistor When the output of the driving voltage source 1 8 appears across grid resistor 16, the loutput being represented by the solid curve eg, the net grid to cathode voltage is the bias voltage developed across the resistor 12 plus the driving voltage. It can be seen that during the first cycle of the driving voltage, the maximum positive swing of the driving voltage would appear to produce a maximum current Io max. represented by the ordinate of point P-Z on the curve C, and the maximum negative swing of this voltage source would appear to produce a minimum current Io-min. represented by the ordinate of point P-3 on the latter curve. If. the output current waveform is traced, the waveform will appear as a curve `Ip. In the example being illustrated, the, power amplifier tube is driven into a nonlinear portion of the curve C, at the lower or cut-off end thereof wherein the bottom half of the plate current waveform produced by the negative half cycle of the drivingvoltage eg will be distorted or flattened. It can' be shown that this non-linear operation` will actually cause the average plate current fiow to increase, for example, to va value of Io', which means that the average cathode bias voltage developed across the resistor12 will increase, accordingly, from' the no-signal condition bias.` Therefore, the median swing of the grid driving voltage eg will change from e--0, the abscissa of point P-l on the curve C, to avalue e-Io' identified by the abscissa of point P74, so, that the negative going swing of the driving voltage source represented by dotted curve eg thenrwill drivev the tube further into its ynon-linear region of operation, closer to a class AB type of operation. The ,maximum swing of the net grid to cathode voltage under the load bias identified by the point P-6 on the curve C. This will result in greater distortion in the output current of the amplifier as shown by the dotted kcurrent waveform Ip. In order to stay within a certain minimum distortion limit, it can be seen that with the arrangement shown in Fig. 1, the maximum swing in the driving voltage permissible for a given distortion level will be much less than is theoretically possible because of the change in the bias level of the tube 2. This means that the permissible amplitude of the output signal of the amplifier for a given distortion limit will be that much less than is theoretically possible. Since power is a function of the square of the output current, it can be readily seen that the power output capabilities of a given tube having the above-mentioned shift in the cathode bias voltage can be substantially less than is theoretically possible in the absence of a stabilization of the bias voltage of the tube 2. Thus, ifsome means were provided for keepingthe cathode bias voltage at a level e-o independently of the arnplitude of the driving voltage, the tube can be driven to its maximum theoretical limit for a -given limiting distortion level.

Before describing the manner in which the present invention stabilizes the grid voltage, reference should be made to Fig. 2 which shows in box diagram form a typical high-fidelity amplifier circuit. Generally, one or more pre-amplifier stages 25 are provided for amplifying the voltage generated in the pick-up head of a phonograph pick-up arm. One typical tube type which can be used in these pre-amplifier stages is the type 12AX7'which has two 6.3 volt filament sections which can be connected in series. These filament sections will be assumed to be in series connected and will be indicated in the drawings as a 'single 12.6 volt filament. The current rating for such 12AX7 filaments is Ain the neighborhood of 150l ma. (It should be understood that the present invention is not limited to a particular tube type, but that Athe type 12AX7 is merely selected for purposes of illustration.)

The output of the pre-amplifier stages 25 is generally fed to what -may be referred toas a control amplier vstage 27. The control amplifier'actually has a relatively s rnall amplification factor and is part of-volu'me and tonev control circuit. This circuit may' use a-type,` 12AV7 tube which has a'l2.6 volt filament circuit also rated at about 150 ma.l The output of the control amplifier 27 leads to the input of a power amplifier stage 29 which, in most cases, would be a push-pull amplifier, but which will now be described as a single tube push-pull amplifier to` exemplify a simple form of the present invention.

As above indicated, becauseof the high amplification of the signals introduced in the pre-amplifier stages, to avoid high amplification of frequencies in the power frequency range, it is desirable. to heat the filaments of the pre-amplifier tubes with direct current, which may be the plate current of the tube or tubes of the power Yam,-

plifier stage 29. Where the power amplifierftubelis a type 6L6, the plate current is in the neighborhood of only 40 to 60 ma. This arrangement is shown in Fig. 3, which additionally incorporates features of the present invention by ymeans ofwhich the bias voltage of the power amplifier tube and also the current flowl through the filaments of the pre-amplifier stages are stabilized at their rated values. p Y' 7 Referringto Fig. 3, the power amplifier circuit 29 there shown is the same as that shownin Fig. 1 except for the cathode circuit, that is the circuit extending between the cathode electrode 2cv and the common or ground point 10.' This cathode circuit includes filaments of the pre-amplifier 12AX7 tubes,ridentifiedby reference numerals 32 and 34, respectively, and resistance means generally indicated by the reference numeral 36. .This resistance means may include the filament 36a Vof the control amplifier tube, which, as above indicated, may be a type 12AV7 tube rated at 12.6 volts at'l50 ma. The resistance means 36 also preferably includes a resistor 36b connected between the filament 36a and the cathode electrode 2c, and also a relatively large resistor 36o connected across the filaments 32, 34 and 36a. A filter capacitor 40 is shown connected in parallel with the preamplifier tube filaments 32 and 34.

To stabilize the cathode bias voltage, an external current or voltage source, generally indicated by the reference numeral 41, is provided which most advantageously provides a direct current Voltage equal to the desired cathode bias voltage. This current or voltage-producing means may, in a broad aspect of the invention, be any source of direct current voltage. Where the external v oltage source has any significant ripple component at an audible frequency, an additional filter capacitor, not shown, may be added across the external voltage source 41. The filter capacitor 40 may be present primarily to avoid cathode degeneration at the signal frequencies, but it also can be provided to aid in filtering any ripple cornponents inthe output of voltage source 41 from the preamplifier filaments. The external voltage source 41 is preferably connected between the cathode electrode 2c orl a point near cathode potential and the common or ground point 10 of the amplifier, and is polarized so'that the current produced thereby in the cathode circuit is in the same direction as the fiow of plate current in the cathode circuit. With this arrangement, voltage source 41 need only supply an increment of current which, in addition to the plate current, will produce a voltage drop across the cathode circuit equal to the desired bias voltage. If the average plate current should decrease for any reason, the external voltage source 41 will supply the necessary current to maintain thecathode bias voltage at the value of the output voltage of the external voltage source 41.

The value ofthe resistors 36h and 36C are selected so that the current fiow in the cathode circuit will be equal to the rated value of the filaments 32 and 34. Where the plate current is less than the current required by the filaments 3 2 and 34, theresistor 36e serves the purpose of providing a current path for plate current if one of Without resistor 36e, the'burning out of one of these filaments would place the full output of voltage source S across the capacitor 40 and the capacitor would, therefore, need to be a high Voltage capacitor.V If the plate current were greater than the current required by these filaments, shunt resistor 36C would be of a value to reduce the plate current flow under both no-signal and signal conditions below that required, so that the external voltage source 4l could be used to regulate both cathode circuit current and voltage.

In the example being described, the pre-amplifier filaments 32 and 34, and the filament 36a for that matter, all require about 150 ma. Where the power amplifier tube is a 6L6 tube whose plate current is in the neighborhood of 55 ma., it can be seen that the current-generat- `.ing means 41 will supply approximately 95 ma. of current.

With the arrangement just described, as soon as the amplifier is turned on, the filaments 32 and 34 and 36a will immediately receive a large surge of current from voltage source 41, so that the warm-up time of the pre-amplifier tubes is not delayed by the warm-up time of the power amplifier tube 2. Moreover, by the means just described, not only is the cathode bias voltage stabilized, but the pre-amplifier filaments 32 and 34 receive their rated current even though, the plate current of the power amplifier is below the current required for optimum operation of the pre-amplifier tubes.

Due to the present invention, at a small additional cost, an amplifier which is normally designed for operation as -a 2O watt amplifier at a given distortion level can produce an output of 35 watts at no greater distortion level and at a very nominal cost. The output level of 35 watts cannot be obtained, however, with tubes now commonly available without a push-pull power amplifier arrangement.

Reference should now be made to Fig. 4 which shows a preferred form of the invention utilizing a push-pull power amplifier 29 using two power amplifier tubes, 2 2, which may be the same 6L6 beam power tubes previously described. In such a circuit, the cathode electrodes 2c -2c of the tubes 2-2 are connected together, the plate electrodes 2a--2a are connected to opposite ends of an output step-down transformer 4', the grid electrodes 2b-2b are connected to respective resistors 16-16 whose common ends are grounded, and the screen grid electrodes Ztl-42d are connected to capacitors 22-22 and resistors 20-20 in the same manner as in the previously described embodiments. The plate voltage 8 for the power amplifier tubes 2-.2 may be derived from a conventional full-wave rectifier circuit operated from a. 60-

cycle power line. The rectifier circuit includes a power rectifier tube 8a whose anode electrodes are connected to the opposite ends of the main secondary winding 8b of a power transformer 8b whose primary winding is connected to a suitable plug connector 8c leading to a 60- cycle power line. The center point of the main secondary winding Sb of the power transformer is grounded. The power transformer further includes one or more filament windings 8b" and 811'", the winding 8b" connecting to the filament of the full-wave rectifier 8a and having a center tap which extends to a conventional filtering network circuit 8d which provides filtered direct current voltage at its output. The secondary winding 8b" may be a filament transformer winding connected to the filaments 2 f-2f of the tubes 2 2. One terminal of the filter network 8d is grounded and the other end is connected to the plate circuits of the various tubes to be used in the amplifier. As shown in Fig. 4, the output of the filter 8d is connected to the center tap point of the input winding of the output transformer 4'. The screen circuit resistors Zit-2li are also connected to this point.

The cathode circuit of the power amplifier is connected between the cathode electrodes 2c--2c of the power amplifier tubes 2-2 and ground. This cathode circuit may be similar to that previously described in connection with the embodiment of Fig. 3 and, there fore, may include in Series circuit relation the pre-amplifier tube'laments 32 and 34 of a tube, such as type 12AX7, and resistancermeans generally indicated by the reference numeral 36, which' may include filament 36a of the control amplifier tube (such as a type 12AV7 tube),- series resistor 36b' and shunt resistor 36e. A filter capacitor 40' is. connected across the filaments 32 and 34.

This push-pull circuit works particularly Well with the tube types now generally available on the market, such as power amplifier tube type 6L6, pre-amplifier tube types 12AX7 and control amplifier tube type 12AV7. The 12AX7 tube has a 12.6 volt filament circuit requiring 150 ma. The combined plate currents of the 6L6 power amplifier tubes is somewhat under 150 ma., so that the current or voltage-generating means to be connected to the cathode circuit for voltage stabilization can supply the differences between the combined plate currents of the power amplifier tubes and the current required by the iilaments'32 and 34.

In order-toH minimize the number of additional components required to fabricate the push-pull power amplifier circuit of the present invention, it is desirable to utilize tap-off points on the main output winding 8b' Vof the power transformer 8b to derive the cathode bias stabilizing voltage. These .tap-ofi points are made at corresponding points 45-45 spaced equi-distant from and onopposite sides of thegrounded center tap point thereof. v Connected to these points are respective semi-conductor type rectifiers 47-47 which are polarized to pass current to the cathode circuit electrodes in the same direction as plate current flows in the cathode circuit. Such rectifiers commonly have relatively low impedances so that the external fixed bias source including the rectifiers yis a -low impedanceV .voltage sourcewith good regulation having a voltage at or near the level at which cathode bias stabilization is desired. Rectifie-rs 47--47 are connected together and then, via lines 48 and 48', to the above mentioned cathode electrodes 2c2c. The full wave rectified voltage occurring at the output of these rectifiers is filtered by a suitable filter capacitor 49 connected between the cathode electrodes and ground. In addition to acting as a filter capacitor, capacitor 49 prevents any appreciable cathode degeneration. To eliminate substantially any cycle component from the pre-amplifier filaments 32 and 34, which would produce hum in the output of the power amplifier, the filter capacitor 40' is also preferably provided. The cathode circuit resistor 36b' is sized so that the current iiowing through the cathode circuit at the desired cathode bias voltage level across lthe cathode circuit is at current or near the rating of the filaments 32 and 34, namely 150 ma. in the above example.

The various parameters of the circuit -in the embodiment of Fig. 4 may be as follows:

It can thus be seen that the present invention provides lat a very slight increase in cost cathode voltage stabilization of the power amplifier in a manner which minimizes Warm-up time and provides the optimum rate of current for the filaments of the pre-amplifier tubes.

4 assaut;

9. It should be understood that numerous modifications may be made in the embodiments of theinvention described without deviating from the broad aspects 'of the invention. For example, where the voltage between the cathode electrode and ground isl stabilized at a value above the desired cathode to grid bias level, the grid circuit may be connected to a tap-ofi point in the cathode to ground circuit which is above ground potential.

We claim as our invention:

l. In an amplifier having pre-amplifier means including electron tube amplifying means with electron-generating filament means and power amplifier means including electron tube amplifying means with an output current vs. signal input characteristic with a non-linear section at the cut-off end thereof and including plate, indirectly heated cathode and control grid electrode means, an output plate circuit connected between said plate electrode means and a common point and including a load device and a source of direct current plate voltage, and a signal input circuit connected between said grid electrode means and said common point,'the improvement comprising: a cathode circuit` connected between said cathode electrode means and said Ycommon point and including said filament means of said preamplifier electron tube means which is arranged to be heated'by the plate current of said power ampl-ifier electron tube means, the voltage drop across said cathode circuit constituting at least part of the grid vtoy cathode bias of said latter electron tube means, and means for stabilizing the cathode bias voltage developed in said cathode circuit so that it is substantially independent of any variation in the average value of the plate current brought about by the driving of said power amplifier electron tube means into said non-linear` region of operation thereof, said stabilizing means including a source of fixed bias voltage having a value near the value of the cathode bias voltage desired and 'connected through a relatively low impedance path to said cathode circuit including said filament means in said cathode circuit for providing the desired bias for said power amplifier electron tube means.

2. In an amplifier having pre-amplifier means including election tube amplifying means with electron-generating filament means and power amplifier means including electron tube amplifying means with an output current vs. signal input characteristic with a non-linear section at the cut-off end thereof and including plate, indirectly heated cathode and control grid electrode means, an output plate circuit connected between said'plate electrode means and a common point and including a load device and a source of filtered and rectified plate voltage, and a signal input circuit connected between said grid electrode means and said common point, a source of `driving signal voltage of varying amplitu-de to be amplified by said power amplifier electron tube means and which drives said latter tube means into said non-linear section of operationthereof, said source of signal voltage coupled to said signal input circuit, the improvement cornprising: a cathode circuit connected between saidvcathode electrode means and said common point and including said filament means of said pre-amplifier electron tube means which is arranged to be heated by the plate current of said power amplifier electron tube means, the voltage drop across said cathode circuit constituting at least part of the grid to cathode bias of said latter electron tube means, means for stabilizing the cathode bias voltage developed in said cathode circuit so that it is substantially independent of any variation in the average value of the plate current brought about by the driving of said power amplifier electron tubemeans into said nonlinear region of operation thereof, said stabilizing means including a source of rectified alternating current voltage coupled through a relatively low impedance path to said cathode circuit including said filament means in said cathode circuit for providing a stabilized bias for said power amplifier electron tube means and for supplying a stabilized current for said filament means, and filter capacitor means coupled across said filament means for substantially eliminating any hum-producingcomponent in the current fed to the cathodeV circuit from said, sources of rectified voltage.

3. In an amplifier having pre-amplifier means including electron tube amplifying means with electron-generating filament means and power amplifier means including electron tube amplifying means with an output current vs. signal input characteristic with a non-linear section at the cut-ofi end thereof and including plate, indirectly heated cathode and control grid electrode means, an output plate circuit connected between said plate electrode means and a common point, and including anl output stepdown transformer and a source of direct current plate voltage, a speaker system coupled to the output of said transformer, and a signal input circuit connected between said Vgrid electrode means and said common point, a source of driving signal voltage of varying amplitude to be amplified by said power amplifier electron tube means and which drives said latter tube means into said non-linear section of operation thereof, said source of signal voltage being coupled to said signal input circuit, the improvement comprising: a cathode circuit connected between said cathode electrode means and said common point and including said filament means of said pre-amplifier electron tube means which is arranged to be heated bythe plate current of saidA power amplifier electron tube means, the voltage drop across said cathode circuit due to plate current of said latter tube means providing less than the desired amount of cathode bias therefor, and the value of the plate current passing through said filament means being less than the rated current therefor, and means for stabilizing the cathode bias voltage developed in said cathode circuit atthe desired cathode biaslevelzso thatv it is substantially'independent of any variation in the aver age value of the plate current brought about by the driving of said power amplifier electron tube means into said non-linear region of operation' thereof and for increasing the level of current owing through said filament means to bring it nearer the rated current value therefor, said stabilizing means including a source of fixed bias voltage coupled, through a relatively low impedance path across said cathode circuit including said filament means.

4. In an amplifier having pre-amplifier means including electron tube amplifying means with electron-generatingfilament means and power amplifier meansincluding electron tube amplifying means with an output current vs. signal input characteristic'with a non-linear section at the cut-off end thereof and including plate, indirectly heated cathode. and co-ntrol grid electrode means, an. output plate circuit connected between said plater electrode means and a common point and including a load device and a source of rectified and filtered alternating current plate voltage, and a signal input circuit connected to said grid electrode means, the'improvement comprising: a cathode. circuit connected between said cathode electrode means and said common point and including said filament means of said pre-amplifier electron tube means which is arranged to be heated by the plate current of said power amplifier electron tube means, means for stabilizing the voltage developed in said cathode circuit so that it issub'stantially independent of any variation in the average value of the plate current brought about by the driving of said power amplifier electron tube means into said non-linear region of operation thereof, said stabilizing means includinga source of rectified alternating current voltage coupled to said cathode circuit so as to send current through said filament means in the same direction as the fiow of plate current therethrough, for, providing the-desired bias for said power amplifier electrontube means, andfilter capacitor means coupled across said filaments for'iiltering therefromv any fluctuatl1 ing audible components in the current resulting from said alternating current voltage source.

5. In an amplifier having pre-amplifier means including electron tube amplifying means with electron generating filament means, and power amplifier means including electron tube amplifying means with an output current vs. signal input characteristic with a non-linear section at the cut-off end thereof andincluding plate, cathode and control grid electrode means, an output plate circuit connected between said plate electrode means and a common point and including an output step-down transformer to be connected to a speaker system and a source of direct current plate voltage, and a signal input circuit connected between said grid electrode means and said cornmon point, a source of driving signal voltage of varying amplitude to be amplified by said power amplifier electron tube means with low distortion for a power output requiring said latter tube means to be driven into said non-linear section of operation thereof, said source of signal voltage being coupled to said signal input circuit, the improvement comprising: a cathode circuit connected between said cathode electrode means and said common point and including said filament means of said pre-amplifier electron tube means which is arranged to be heated by the plate current of said power amplier electron tube means, the voltage drop across said cathode circuit supplying substantially all of the grid-to-cathode bias of said latter electron tube means, means for stabilizing the cathode bias voltage developed in said cathode circuit so that it is substantially independent of any variation in the average value of the plate current brought about by the driving of said power amplifier electron tube means into said non-linear region of operation thereof, said stabilizing means including a source of alternating voltage of a frequency in the audible range and having a value which when rectified and filtered will provide the desired cathode bias voltage for effecting the desired operation of the latter electron tube means, low impedance rectifier means, said source of alternating voltage being coupled through said rectifier means across said filament means, and filtering means in said cathode circuit for filtering out humproducing components from said cathode circuit originating from said source of alternating voltage.

6. In an amplifier having pre-amplifier means including electron tube amplifying means with electron-generating filament means and power amplifier means including electron tube amplifying means with an output current vs. signal input characteristic with a non-linear section at the cut-off end thereof and including plate, cathode and control grid electrode means, an output plate circuit connected between said plate electrode means and a common point and including an output step-down transformer to be connected to a speaker system and a source of direct current plate voltage, and a signal input circuit connected to said electrode means, the improvement comprising: a cathode circuit connected between said cathode electrode means and said common point and including said filament means of said pre-amplifier electron tube means which is arranged to be heated by the plate current of said power amplifier electron tube means, resistance means connected in series with said filament means, means for stabilizing the voltage developed in said cathode circuit so that it is substantially independent of any variation in the average value of the plate current brought about by the driving of said power amplifier electron tube means into said nonlinear region of operation thereof, said stabilizing means including an external source of alternating voltage of a frequency in the audible range, said source of alternating voltage being coupled through said rectifier means across said cathode circuit, and filter capacitor means in said cathode circuit coupled across said filament means for filtering out hum-producing components from said cathode circuit originating from said source of alternating voltage, and said resistance means being of a value that the current fiowing through said filament means for the voltage applied to the cathode circuit by said external source of voltage is near the rated value of said filament means.

7. In an amplifier having pre-amplifier means including electron tube amplifying means with electron-generating filament means and power amplifier means including electron tubeV amplifying means with an output current vs. signal input characteristic with a non-linear section'at the cut-off end thereof and including plate, cathode and control grid electrode means, an output plate circuit connected between said plate electrode and a common point and including an output step-down transformer to be connected to a speaker system and a source of direct current plate voltage, and a signal input circuit, the improvement comprising: a cathode circuit connected between said cathode electrode means and said common point and including said filament means of said pre-amplifier electron tube means which is arranged to be heated bythe plate current of said power amplifier electron tube means, first resistance means connected in series with said filament means and second resistance means connected in parallel with said filament means, means for stabilizing the voltage developedin said cathode circuit so that it is substantially independent of any variation in the average value of the plate current brought about by the driving of said power amplifier electron tube means into said non-linear region ofoperation thereof, said stabilizing means including an external source of voltage, and said resistance means being of a value that the current flowing through said filament means for the voltage applied to the cathode circuit by said external source of voltage is near the rated value of said filament means.

8. Inran amplifier having pre-amplifier means including electron tube amplifying means with electron-generating filament means and power amplifier means including electron tube amplifying means with an output current vs. signal input characteristic with a non-linear section at the cut-off end thereof and including plate, cathode and control grid electrode means, an output plate circuit connected between said plate electrode means and a common vpoint and including an output step-down transformer to be connected to a speaker system and a source of direct current plate voltage, and a signal input circuit connected Yto said grid electrode means, the improvement comprising: a cathode circuit connected between said cathode electrode means and said common point and including said filament means of said pre-amplier electron tube means which is arranged to be heated by the plate current of said power amplifier electron tube means, first resistance means connected in series with said fialment means and second resistance means connected in parallel with said filament means, the voltage drop across said vresistance means and filament means constituting most vof the grid-to-cathode bias of said latter electron tube means, meansV for stabilizing the cathode bias voltage developed in said cathode circuit so that it is substantially (independent of any variation in the average value of the plate current brought about by the driving of said power amplifier electron tube means Vinto said non-linear region of operation thereof, said stabilizing means including an external source of alternating voltage of a frequency in the audible range and having a value which when rectified and filtered will providerthe desired cathode bias ,voltage for the latter electron tube means, semi-conductor Yrespectively coupled acrossthe entire cathode circuit and across said filament means, for filtering out hum-producing v components from said cathode circuit originating from said source of alternating voltage, and said resistance means being of a value that the current flowing through said filament means for the voltage applied to the cathode circuit by said external source of voltage near the rated value of said filament means.

9. In a high-fidelity sound amplilier Ahaving preamplier means including electron tube amplifyingk means l with electron-generating filament means and power amplier means including electron tube amplifying means with an output current vs. signal input characteristic with a non-linear `section at the cut-oli endthereof and including plate, indirectly heated cathode and control grid electrode means, an output plate circuit connected be-L tween said plate electrode means' and common point and including load means including a speaker system and a circuit connected between said grid electrode means and said common point, a source of driving signal ,voltage of varying amplitude to be Vampliied by said power amplier electron tube means with minimum distortion for a power output requiring said latter tube means to be driven into said non-linear section of operation thereof,

said source of driving signal voltage being coupled to said signal input circuit, the limprovement comprising: a cathode ycircuit connected between said cathode electrode means and said common point and including said filament means of said pre-amplifier electron tube means, the normal average value of the plate current of said power amplier electron tube means Ybeing below the recommended value of current for said pre-amplifier electron tube means and the. voltage drop across said cathode circuit due to ow of plate current of said vpower f ampliiier electron tube means therein constituting less than the desired amountof cathode bias forthe latter device, an external bias-stabilizing voltage means connected to said cathode circuit rfor providing current tlow in said cathode circuit which s in the same direction as the direction of plate current flow therein and which raises the total voltage drop in said cathode circuit to the desired voltage level, the total 4current owing through said filament means being in .the neighborhood of the recommended current lfor said lamentf means.

l0. In an amplier having pre-amplifier means including an'electron tube amplifyingA means with electrongenerating filament means andpower amplier means inlsource of direct current plate voltage, and a signal input current vs. signal input characteristic having a nonlinear section at the cut-off end thereof and including plate, indirectly heated cathode and control grid electrode means, an output circuit connected between said plate electrode means and a common point and including a load `deviceand a source of direct current plate voltage, and

ra signal input circuit connected to said control grid electrode means, the improvement comprising: :a cathode circuit connected between said cathode electrode means and ,said common point and includingv said filament lmeans of said preamplifier electron tube means which is to said cathode circuit including said filament means in Y said cathode circuit for providing the desired bias for cluding electron tube amplifying means with an output said power ampliiier electron tube means.

References Cited in the le of this patent UNITED STATES PATENTS 2,259,471 Lfarms oct. 21, 1941 2,542,293 smith Feb. 2o, 1951 2,759,050 Lard` Aug. v14, 1956 t FOREIGN PATENTS l 281,319 Great'ritain Aug. 16, V1928 716,873 Great Britain Oct. 13, 1954v OTHER REFERENCES Smith: Heater Supplies for Hum Reduction in Audio Engineering, August 1948, pages 2628 '(particularly rst paragraph middle column of page 26).

R.'C.A. Receiving Tube Manual, published by the Tube Division ofl R.C.A. of America, Harrison, NJ., copyright1956, page 321 Fig. 18-2 relied upon. 

